Such a method is known inter alia from U.S. Pat. Nos. 4,519,995 and 4,559,211, in both of which powderlike lime is fed by injection into the furnace of a boiler, wherein the lime is converted to oxide under the influence of heat then reacts with the sulphur oxides contained in the flue gases. The reaction results in the formation of calcium sulphite and calcium sulphate in dust form. This dust is carried with the flow of flue gases into dust separators, in which fly ash and other dustlike components are removed from the flue gases before the flue gases are discharged into the atmosphere through a chimney. Both these solutions have the drawback that the lime is blown in the boiler to a point determined by the structure of the boiler and geometrical considerations, as a result of which it is impossible to control the reaction conditions after blowing. After blowing the lime dust proceeds at a rate determined by the gas flow in the boiler and at a temperature prevailing therein, wherefore the process, though apparently systematic, is chemically uncontrollable so that its desulphuration capacity is partially arbitrary. Further, the process can be regarded as uncontrollable because the time the the lime is present in the boiler and, as a consequence, the conversion of lime to oxide, the calcination process and the reaction of oxide with sulphur, for example, are solely designed to meet the thermic requirements set for the boiler, whereas sulphur purification reactions have not been taken into account in the design.
For these reasons, prior art methods of desulphuration do not provide the best possible desulphuration effect as both the desulphuration capacity and economy of the prior art methods are insufficient, and the achievement of the required effect is technically complicated as well as expensive.